1. Field of the Invention
The present invention generally relates to front-end rectifiers with power-factor correction (PFC). In particular, the present invention relates to three-phase, three-level PFC rectifiers.
2. Discussion of the Related Art
In power converters, achieving a high efficiency in high-voltage applications is a major design challenge that requires an optimized reduction of conduction and switching losses through a careful selection of the converter topology and switching device characteristics. Specifically, a higher voltage-rated semiconductor switch exhibits larger conduction and switching losses, as compared to a counterpart with a lower voltage rating. In this context, a semiconductor switch may be any switching device, such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated-Gate Bipolar Transistors), a BJT (Bipolar Junction Transistors), a SiC (Silicon-Carbide) or a GaN (Gallium-Nitride).
Generally, switching losses can be reduced and even eliminated using resonant or soft-switching techniques. However, there are only limited approaches for reducing conduction losses. In fact, once the topology and the switches with the lowest conduction losses for the required voltage rating are selected, further decrease in conduction loss is possible only by modifying the topology to utilize switches with a lower voltage rating. Multilevel converters—whose switches operate with a voltage stress that is much less than the input and output voltages—are naturally suitable for high-voltage applications.
The Copending Application describes a new, three-phase, two-switch, zero-voltage switching (ZVS), discontinuous conduction mode (DCM), PFC boost rectifier that achieves a low input-current total harmonic distortion (THD). In that PFC boost rectifier, all the switches operate under ZVS conditions, without using additional soft-switching circuitry. One implementation of the PFC rectifier of the Copending Application is shown in FIG. 1. As shown in FIG. 1, the PFC boost rectifier includes Y-connected capacitors C1, C2, and C3, which create virtual neutral node N. Virtual neutral node N has the same electrical potential as the power source's neutral terminal that is not physically available for connection in a three-wire power system. Since virtual neutral node N is connected to the node between switches S1 and S2 and also to the node between output capacitors CO1 and CO2, the electrical potentials of these nodes are the same as the electrical potential of the neutral terminal in the balanced three-phase power source.
In addition, by connecting virtual neutral node N directly to the node between switches S1 and S2, decoupling of the three input currents is achieved. In such a decoupled circuit, the current in each of boost inductors L1, L2 and L3 depends only on the corresponding phase voltage, which reduces the THD and increases the power factor (PF). Specifically, bridge diodes D1-D6 allow only the currents in phases with positive phase voltages to flow through switch S1, when switch S1 is turned on, and allow only the currents in phases with negative phase voltages to flow through switch S2, when switch S2 is on. Therefore, the boost inductor corresponding to a phase in a positive voltage half-line cycle carries positive current when switch S1 is on, while the boost inductor corresponding to a phase in a negative voltage half-line cycle carries negative current when switch S2 is on. During the time when switch S1 is off, the stored energy in the boost inductor connected to the positive phase voltage is delivered to capacitor CR, whereas the stored energy in the boost inductor connected to the negative phase voltage is delivered to capacitor CR during the time when switch S2 is off. Because the voltage between either terminal of capacitor CR and virtual neutral node N abruptly changes with a high rate (i.e., a large dV/dt value) during each switching cycle, coupled inductor LC is connected between “flying” capacitor CR and the output voltage VO to isolate output voltage VO from these fast high-voltage transitions that usually produce unacceptable common-mode electromagnetic interference (EMI) noise. As shown in FIG. 1, with coupled inductor LC, the node between output capacitors CO1 and CO2 can be directly connected to virtual neutral node N, which makes the output common-mode noise very low. Moreover, because of coupled inductor LC, parallel operations of multiple rectifiers are also possible.
To facilitate cross-reference between the figures and the detailed description, like elements are assigned like reference names or numerals.